In view of the potential application of HBOCs in perfusion of organs for cardioplegia during major surgery, or for preservation at low temperatures before transplant, we examined temperature dependence profiles of oxygen equilibrium curves (OECs) at 15-37 degree c for a number of human and bovine crosslinked hemoglobins. OECs for these hemoglobins were carried out on the HEMOX-ANALYZER fitted with a thermostated cell. OECs for modified hemoglobins at 37 degrees c were right shifted as compared to unmodified hemoglobin. Lowering the temperature however, resulted in increased affinity towards oxygen. It shifted OEC to the left. However bovine hemoglobin crosslinked at the beta-beta subunits exhibited significantly different and more favorable oxygenation at very low temperature. Such is the case with a newly developed modified human hemoglobin crosslinked with a pyridoxyl tetraphosphate derivative. This led us to believe that not only the site of modification, but also the nature of the chemical modification plays a crucial role in determining the degree of conformational constrains placed upon the hemoglobin when loading and unloading oxygen. This will ultimately determine its usefulness as a perfusate under hypothermic conditions. Kinetic characterization of binding to these hemoglobins is now underway using fast reaction techniques in an attempt to relate both equilibrium and kinetic manifestations of hemoglobin function under hypothermic conditions. This work was presented at the IV International Symposium on Blood Substitutes (August 19-23, 1991, Canada.